The number of revolutions (RPM) of a diesel engine has been conventionally counted on the basis of a detected signal by a mechanical detector or magnetic detector fitted to an output shaft of the diesel engine or to a cam driving shaft of a fuel injection pump rotating in proportion to the output shaft. However, in the respective cases, it is difficult to retrofit the detector to the output shaft or to a cam driving shaft when the diesel engine has been incorporated in an automobile or the like.
Among the vibration components propagated through the fuel injection pipe of a diesel engine is a component generated at the end of the injection cycle. This component, generated by a needle valve within the nozzle of a cylinder associated with the fuel injection pipe, has an amplitude larger than that of the other vibration components. Therefore, when the vibration propagated through the fuel injection pipe is converted to an electric signal, only the component of the signal obtained from the vibration the end of injection need be counted. In this case, the detecting means may be fitted to the fuel injection pipe, and therefore, a simpler installation may be obtained then in the conventional case, where the detector is fit to a rotary shaft.
However, according to experiments made by the present inventor, the number of revolutions could be accurately counted only at the time of low engine revolution and not at the time of high engine revolution. This is because at the time of high engine revolution, the vibration components propagating through the fuel injection pipe not only include the amplitude of the vibration component (called a "signal vibration component") generated at the end of the injection within the nozzle of the cylinder corresponding to the above-mentioned fuel injection pipe but also the amplitude of the vibration component (called a "noise vibration component") generated at the end of the injection within the nozzle of another (i.e. adjacent) cylinder. The latter vibration might become larger than the first mentioned predetermined level, and therefore, the noise vibration component might also be sensed as a signal vibration component.